This invention relates to foam structures comprising a plurality of hollow, coalesced foam strands, their preparation and use, and extrusion foaming dies suitable for use in fabricating such structures. This invention also relates to foam structures that include both hollow, coalesced foam strands and solid, coalesced foam strands together with their preparation and use, and extrusion foaming dies suitable for use in fabricating such structures.
A variety of patent publications relate to foamed objects that comprise a plurality of coalesced, distinguishable, expanded (foamed) polymer strands (strand foams). Illustrative publications include U.S. Pat. No. 3,573,152 (column 2, lines 19-35, column 2, line 67 through column 3, line 30, column 4, line 25 through column 5, line 19 and column 5, line 64 through column 6, line 46); U.S. Pat. No. 4,801,484 (column 1, lines 12-21, column 2, line 55 through column 5, line 8, column 5, lines 16-50 and column 5, line 60 through column 6, line 6); U.S. Pat. No. 4,824,720 (column 2, lines 57-68, column 3, line 57 through column 5, line 32 and column 5, lines 50-58); U.S. Pat. No. 5,124,097 (column 3, line 34 through column 4, line 3, column 5, line 31 through column 6, line 11 and column 6, lines 36-54); U.S. Pat. Nos. 5,110,841; 5,109,029 and; European Patent Application (EP-A) 0 279,668; Japanese Patent Application numbers (JP) 60-015114-A; 53-1262 and H6-263909. The relevant portions of such publications, especially those specifically noted, are incorporated herein by reference.
In general terms, strand foam preparation includes extrusion of a foamable material, typically a thermoplastic polymer material, through a multi-orifice die plate to generate individual foam able strand elements. The strands expand and coalesce (e.g. proximate strands contact each other along at least a surface portion of their respective lengths while the strands retain sufficient surface tackiness to effect strand-to-strand adhesion) after emerging from the die plate, yet remain distinguishable following recovery of the strand foam. Typical strands have a circular cross-section where the die orifices or apertures are circular. Altering the orifice shape to a slot, a square or a special shape leads to a corresponding partial alteration in strand cross-section. Partial alteration results from a tendency of strands exiting a die orifice to round with foaming rather than hold true to an aperture shape.
Strand foams offer a number of performance advances. For example, strand foams have excellent strength in a plane transverse to the direction of extrusion. They provide a predetermined shape with little or no need to trim. They function as low density products with distinguishable coalesced cellular strands. With the ability to change or alter die orifice shape and arrangement, strand foams readily adapt to variations in shape.